1. Field of the Invention
The invention relates to a method and a device for the production of a printing block. The printing block may, for example, be a flexible or inflexible printing block, which can act as a relief printing or gravure printing block.
2. Description of the Relevant Art
To produce the flexographic printing block with the aid of a conventional CO2 laser it is already generally well known for material to be burned out directly from a printing plate, which may be a polymer plate for instance, in order in this manner to produce a relief in the printing plate. In this process, the CO2 laser is permanently power-modulated to obtain recesses bounding the relief in the surface of the printing plate.
Furthermore, for the production of a flexographic printing block PCT/EP96/05277 already discloses the use of two laser beam sources in order with the first laser beam source to obtain fine structures in a desired profile, while by means of the second laser beam source lower-level regions in the profile are produced.
The state of the art further includes methods for placing small raster dots in a relief at a lower level. This is done in that focused beams staggered close beside one another strike corresponding regions and remove the material in conformity with the focused course of the beams. This then gives rise to a sort of cone whose conical apex is located at a greater or lesser depth in the relief. If in subsequent printing an add-on is arranged under the printing block, that is to say a kind of underlay, then due to this underlay the tip of the cone is lifted back again into the region of the print area. However, printing material adheres quite poorly to this cone tip so that a less than sharp printed image results. Cone tips representing raster dots of this kind are provided by way of example in the vicinity of full print areas so that in subsequent printing the full print areas may be given more prominence. In subsequent printing the said underlay comes to lie beneath a full print area so that during printing a high contact pressure is obtained. Where the depth of the raster dots surrounding the full print area not reduced in advance the latter would press too heavily against the subsequent print area and buckle which would likewise adversely affect the printed image.
It is an object of the invention to specify a method for the production of a printing block, in particular a flexographic printing block, with which fine relief structures to be given prominence in subsequent printing may be produced in such a way that they result in a flawless printed image. Moreover, a corresponding device for producing such printing blocks is to be provided.
In a method according to the invention for producing a printing block, in particular a flexographic printing block, a relief is introduced into the surface of a blank of the printing block in that material of the printing block blank is removed in regions along tracks by radiation in order by this means to form recesses between which plateaus come to lie. Now, according to the invention the surface of the printing block blank located between the recesses is also removed by radiation in such a way that as a result lower-lying plateaus are obtained.
Thus, contrary to the most recently described state of the art fine raster dots later to be given prominence in the relief are not produced in that due to conical and closely adjacent beams more or less low-lying cone tips are blocked in the relief, but rather in that initial plateaus between the respective recesses located initially in the surface of the printing block blank are lowered in depth more or less uniformly in order to obtain lower-lying plateaus whose plateau surface comes to lie as before more or less parallel to the surface of the printing block blank. If, during subsequent printing, these plateaus are lifted, that is to say lifted into the print area, then sufficient printing material remains adhering to them to yield a sharp printed image. This procedure is used when, for example, a relatively large full print area is surrounded by a fine raster so that the full print area is given more prominence.
According to a refinement of the invention, in order to set the depth of the lower-lying plateaus the surface of the printing block blank located between the recesses can be removed by radiation whose intensity or power can be correspondingly adjusted. Thus, if the plateaus lying between the recesses are to be burned away to a greater depth the intensity or power of the beam must be increased and vice versa.
According to another refinement of the invention, in order to set the depth of the lower-lying plateaus the surface of the printing block blank lying between the recesses can also be removed by repeated irradiation. Thus, this multiple irradiation of the printing block blank in the region of the plateaus to produce the lower-lying plateaus ensues with a time delay or successively so that a lower-lying plateau is obtained as it were by repeated scooping out.
Since the lower-lying plateaus of the relief structure are carved out by repeated exposure to radiation or burning off the power of the beam can be relatively low which has the consequence that even very fast modulators, precisely whose beam power when used has to be limited in order to save the modulators from destruction, acousto-optical modulators for instance, can be used for switching the beam power on and off. Due to repeated and hence relatively gentle erosion of the plateau it is also achieved that after each removal operation the printing block material cools again before removal of material starts afresh which has the result that the printing block material in the region of the plateau does not heat up so much and hence the relief can be built up in decidedly exact manner or true to shape. Between the individual burn-off operations the material stripped off can also be taken away, eg sucked off, which allows more precise working in the next removal operation and results in structures of better quality.
In doing so the irradiation of the plateaus can ensue along a particular track using one and the same beam which is guided repeatedly along a track. However, it is also possible for irradiation along a track to be done using a plurality of beams which are conveyed one after the other along the same track. For this purpose it is possible in principle for a plurality of stations to be arranged beside one another in a direction running transverse to the longitudinal direction of the track when a corresponding relative shift between track and beams ensues. However, a plurality of beams located alongside one another in a direction running in the longitudinal direction of the track may also be used.
According to a refinement of the invention the depth of the lower-lying plateaus may be set differently as a function of their position in the relief. Thus, by way of example the depth of the lower-lying plateaus may increase in the direction towards a full print area located in the surface of the printing block blank in order to ensure that during subsequent printing the lower-lying plateaus in the vicinity of the full print area are lifted just into the print area when an add-on or underlay is located under the full print area.
It should be pointed out that the recesses in the surface of the printing block blank present between the plateaus may also be constructed by multiple irradiation of the surface of the printing block blank. This multiple irradiation of the printing block blank to produce the lower-lying recesses then occurs with a delay or successively so that a lower-lying recess is obtained as it were by repeated scooping out. However, the recesses could also be obtained by appropriate control of the power of the beam over the region of a recess.
In a further development of the invention the exposure of the printing block blank to radiation is done using laser radiation since in this manner the requisite radiation energy can be readily made available. In this respect focused laser radiation may be used.
In order to machine the printing block blank along the tracks the beams or laser beams may be moved relative to the printing block blank or this is done in such a way that the printing block blank is moved relative to the fixed beams. Alternatively, the beams and the printing block blank can both be moved relative to one another.
In doing so a printing block blank is used, for example, which has an elastic material forming a printing surface, polymer material, silicone or rubber for instance.
Thus, for example a plate-like printing block blank composed of polymer material or other suitable elastic material can be laid onto the surface of a rotatably mounted cylinder and there be fitted firmly in place, for instance by clipping on, by suction by means of vacuum, by magnets, etc. However, to form a printing block blank elastic or polymeric material may also be drawn onto or applied to a rotatably mounted cylinder. For example, these can be flexible tubes which are drawn onto the cylinder or liquid material or polymer material can be applied by knife coating, spraying and immersion, etc.
According to a very advantageous refinement of the invention the exposure of the printing block blank to radiation along the track in question takes place as a function of a data file which each is assigned to the plateaus lying between the recesses. Thus, the removal of the layers of material on the printing block blank in the region of the plateaus occurs under purely digital control so that changes in the radiation power or switch-on/switch-off operations may be carried out very rapidly. At the same time data files can likewise be used to form the recesses lying between plateaus which can also be combined with the data file first mentioned to form an overall file in such a way that the data files form, as it were, links in a chain which are successively worked through.
In doing so the respective files are used for modulating the beams or switching them on and off. These data files could be used for example to control acousto-optical modulators with the aid of which the beams or laser beams are switched on and off and whose mode of operation is known.
In order to allow beams of differing intensity to pass through the acousto-optical modulators can be actuated by different control voltages. In that respect different control voltages may be assigned to the respective data files for modulating the beams in order when using one of the data files in question to use one of the control voltages in question to actuate a modulator. The control voltage in question is then switched on in conformity with the data file. This switched control voltage is then applied to the modulator.
To generate the control voltage passed to the modulator a fast digital-analogue converter, for example, may be used which can, for example, be an 8-bit converter. A digital value of zero would yield the control voltage 0, while a digital value between 1 and 255 would deliver a control voltage of correspondingly set level to the modulator. However, it is also possible to switch a preset control voltage by means of an analogue switch, wherein a data file having only the values 0 and 1 is applied to the control or switching input port of the analogue switch.
A device according to the invention for producing a printing block, in particular for producing a flexographic printing form, contains a mounting for holding a printing block blank, an optical device for irradiating a surface of the printing block blank along a track by means of at least one beam in order by this means to remove material from regions of the printing block blank to form recesses, and a control device which making use of a data file containing beam-on and beam-off switching commands controls changes in the intensity of the at least single beam on its way along the track. This device distinguishes itself in that the control device is constructed in such a way that it makes available at least one data file each containing beam-on and beam-off switching commands in order also to remove by radiation the surface of the printing block blank lying between the recesses so that by this means lower-lying plateaus are obtained.
Thus, with the aid of the device it is possible to obtain relatively small plateaus at a lower level with respect to the original surface of the printing block blank whose plateau surface is as before practically parallel to the original surface of the printing block or concentric with the latter if this should be arched. Thus, the lower-lying plateaus are no longer restricted to regions in the shape of a cone tip but rather extend over an area so that printing material (ink, paste and the like) adheres better thereto giving rise to a high-grade printed result.
In doing so, according to a refinement of the invention the optical device is constructed in such a way that it emits at least one beam, the control device being constructed in such a way that one beam in each case passes through one and the same track several times and on each passage of the track data file or a new data file can be read out. If, for example, only one beam is present and if the original plateaus are to be peeled off or burned off in a plurality of successive stages the beam would have to pass through any track in question a corresponding number of times.
It is also possible, however, that the optical device emits a plurality of beams which are each controllable by a separate data file. In this case all beams would have to traverse one and the same track one after the other.
For this purpose the beams may be arranged alongside one another in a direction running transverse to the longitudinal direction of the track so that as a result of appropriate displacement in the transverse direction the beams can be brought into alignment with the track one after the other. Alternatively, however, the beams may be arranged beside one another in a direction running in the longitudinal direction of the track. In this case the beams are actuated by the data files with a time delay which corresponds to the spacing of the beams in the longitudinal direction of the track.
The beams used may be focused beams, focused laser beams for instance.
In principle the printing block blank can be a plate-shaped blank or a cylindrical printing block blank. It is of elastic construction at least on its surface and is preferably composed of polymeric material or contains at least one such. However, it may also be composed of silicone, rubber or another material, metal for instance.
For machining the printing block blank when constructed in the form of a plate the latter can be machined, for example, in the flat state when beams are guided along tracks and kept at a distance parallel to it. The beam sources and printing block blank could then be displaced relative to one another in parallel planes.
According to an advantageous refinement of the invention the printing block blank is constructed as a cylinder mounted to rotate about its longitudinal axis which carries on its surface an elastic material, for example polymeric material. This can be of plate-like construction and be laid around its surface. If it is fastened in the form of a plate on the cylinder surface the plate can also be removed from the latter again after machining in order to be used as a flat printing plate. However, the elastic or polymeric material may also remain fixed on the surface of the cylindrical support after it has been drawn onto the latter or applied in a different form, for instance by an immersion, knife-coating or spraying process and the like. In this case the entire cylinder is later used as a printing cylinder.
When machining or irradiating the printing cylinder to produce the surface relief the latter can be turned while at the same time a carriage carrying at least parts of the optical device and arranged displaceably in the direction of the longitudinal axis of the cylinder is moved. Items present on this carriage may be, for instance, tilted mirrors for diverting laser beams or laser beam sources may be mounted directly on it. It is also possible when turning the cylinder about its longitudinal axis to displace the latter simultaneously also in the direction of its longitudinal axis so that the surface of the printing block blank can be machined by an optical device in a fixed position. This variant would be advantageous if the optical device itself is composed of a large number of beam sources for producing a large number of beams and hence maladjustment due to vibrations is relatively great.
It has already been mentioned that for control of intensity or control of power, that is for switching the beams on and off, modulators are provided which are actuable via the data files. In doing so these can preferably be acousto-optical modulators which are actuable at high speed.
At the same time a particular one of the modulators is connected to at least one analogue switch through which a control voltage corresponding to the pattern information can be fed to the modulator, wherein the analogue switch can be switched by the data file. By this means very precise digital control of the machining beam or laser beam is possible.
Thus, for example, according to a refinement of the invention a modulator can be connected to the outputs of a plurality of analogue switches which are each switchable by one of the plurality of data files (pattern information) needed for engraving along a track, wherein the analogue switches each switch different control voltages. Depending on the data file and hence the selected analogue switch, a different control voltage corresponding to the pattern information arrives in this way at the modulator so that depending on the selected control voltage the latter emits a beam having greater or lesser intensity or power.
According to another refinement of the invention, however, a plurality of modulators may also be present to each of which an analogue switch is assigned which are each switchable by one of the plurality of data files needed for engraving along a track, wherein the analogue switches each switch different control voltages.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and of the scope of the invention will become apparent to those skilled in the art form this detailed description.